1. Field of the Invention
This invention relates to a method and apparatus for soldering printed circuit boards, and to a cooling mechanism for use within a soldering apparatus to cool solder which has been applied to printed circuit boards by the soldering apparatus.
2. Description of the Related Art
In general, Pb-63Sn, which is a eutectic solder, is used as a solder alloy for the soldering of electronic components printed circuit boards. This eutectic solder has a low liquidus temperature at which it melts of 183xc2x0 C., so when soldering of printed circuit boards is carried out using molten solder in a solder bath, the temperature of the molten solder is generally around 210xc2x0 C. At this temperature, the molten solder has little thermal effect on printed circuit boards or on electronic components mounted on the printed circuit boards.
However, eutectic solder is not suitable for use with all printed circuit boards. Some electronic components which are mounted on printed circuit boards, such as power transistors and high-voltage coils, generate significant heat during their operation and reach a high temperature which can melt or weaken soldered joints made of eutectic solder. To prevent thermal damage of this type, high-temperature solders which will not melt or weaken when exposed to high temperatures from electronic components are often used to solder electronic components having a high power consumption to printed circuit boards.
Typical high-temperature solders are lead-based solders to which small amounts of tin, silver, antimony, etc. have been added, and tin-based solders to which small amounts of lead, silver, antimony, etc. have been added. As indicated by their name, high-temperature solders have a melting point and particularly a high liquidus temperature which is generally at least 220xc2x0 C. With such a high liquidus temperature, the soldering temperature for high-temperature solders must typically be at least 250xc2x0 C., so a bath containing molten solder of this type cannot be used for soldering of printed circuit boards on which heat-sensitive electronic components are mounted.
Recently, the use of lead-free solders has been studied to avoid the problems of lead pollution. Lead-free solders contain tin as a main component, to which is added silver, copper, nickel, or other metals with a high melting point. If only such high-temperature metals are added, the melting point of the solder may be too high, so metals with a low melting point such as bismuth or indium may also be added to lower the melting point of lead-free solders.
The addition of metals such as Bi and In to lead-free solders does in fact lower their solidus temperature, but the liquidus temperature does not significantly decrease and remains at a high level of at least 200xc2x0 C.
The temperature for soldering is generally 20-50xc2x0 C. above the liquidus temperature of a solder, so even if the solidus temperature of lead-free solder is lowered, the soldering temperature is still a high level of at least 220xc2x0 C. Accordingly, even with lead-free solders, it has not been possible to perform soldering of printed circuit boards equipped with heat-sensitive components using a molten solder bath.
A degradation in the properties of heat-sensitive electronic components usually requires prolonged exposure to high temperatures. Exposure to high temperatures will not result in degradation provided the exposure is for a short period. Therefore, thermal damage to heat-sensitive components during the soldering of printed circuit boards can be avoided by completing soldering in as short a time as possible, by individually soldering the components by hand to minimize thermal effects, or by cooling the solder as rapidly as possible after the completion of soldering.
There exist soldering apparatuses in which a fluxer, a preheater, a molten solder bath, a cooling mechanism, and other processing equipment are arranged in series. The cooling mechanism employs a fan to cool printed circuit boards after soldering by blowing air at the printed circuit boards. However, the purpose of cooling by the cooling mechanism is to cool the printed circuit boards themselves and not to prevent thermal effects on heat-sensitive electronic components mounted on printed circuit boards. In actuality, a fan-type cooling mechanism for cooling printed circuit boards cannot cool faster than 10xc2x0 C./second, no matter how cold is the air that it blows, so when heat-sensitive electronic components which have been soldered at a high temperature are cooled with a fan-type cooling mechanism, the performance of the components ends up deteriorating due to prolonged exposure to high temperatures.
The present invention provides soldering technology which does not cause deterioration of electronic components even when soldering is carried out in a molten solder bath using high-temperature solder or lead-free solder.
The present inventors found that if a printed circuit board is rapidly cooled by a cooling liquid after soldering, not only does the rapid cooling prevent the adverse effects of heat, but it also refines the metal matrix of the solder to improve its strength.
The present invention provides a soldering method for printed circuit boards in which molten solder is applied to a printed circuit board, and then the printed circuit board is made to contact a cooling liquid to rapidly cool the solder.
The present invention also provides a soldering apparatus for printed circuit boards equipped with a molten solder bath and a cooling mechanism which can contact a printed circuit board with a cooling liquid after the application of molten solder to the printed circuit board.
The present invention additionally provides a cooling mechanism for use with a soldering apparatus. The cooling mechanism includes a tank for containing a cooling liquid and a discharge port for discharging the cooling liquid at a printed circuit board. In a preferred embodiment, the cooling mechanism is equipped with a chiller which can cool cooling liquid from the tank.
When soldering is performed with a conventional soldering method using high-temperature solder or lead-free solder, if the solder which has been applied to a printed circuit board is exposed to heat cycles of alternating high and low temperatures, the solder undergoes metal fatigue, and cracking and peeling of the solder may occur. However, according to the present invention, the metal matrix of the solder is refined by rapid cooling which takes place after soldering, so the strength of the solder is increased, and metal fatigue due to heat cycles is prevented.
Many high-temperature lead-based solders and lead-free, tin-based solders have a large solidification region, which is the region between their solidus temperature and their liquidus temperature. When soldering is performed using a solder with a large solidification region, it takes a long time for the solder adhering to printed circuit boards to completely solidify. During this period of solidifying, if the printed circuit board is subjected to vibrations or shocks, cracking or peeling of solder which is in the process of hardening can take place, resulting in defective soldered connections. However, according to the present invention, cooling of solder can take place at an extremely high rate, so the occurrence of such cracking and peeling is extremely rare.
Lead-free solder has poorer solderability than eutectic solder. This is due not only to the properties of lead-free solder itself but also to the fact that if soldering of a tin-based solder is carried out in the air, the solder becomes easy to oxidize, and its solderability worsens. Therefore, soldering using a lead-free solder is preferably carried out in an atmosphere without oxygen, i.e., in a nonreactive gas atmosphere. However, in this case, since it is not possible to introduce fresh cooling air from outside a chamber in which soldering is taking place, a fan-type cooling mechanism which simply blows gas within the chamber at printed circuit boards cannot perform adequate cooling. As a result, the above-described problems of cracking and peeling due to inadequate cooling occur. In contrast, according to the present invention, printed circuit boards can be rapidly cooled even in a closed chamber, so cracking and peeling of solder do not occur.